Slot aerials



Sept. 4, 1956 T. R. BOYS ET AL 2,762,044

SLOT AERIALS Filed Aug. 21, 1952 i ZZ Inventor T. R J.

BOYS- United States Patent SLOT AERIALS Application August 21, 1952,Serial No. 305,658 Claims priority, application Great Britain August 21,1951 2 Claims. Cl. 343-767) The present invention relates to slotaerials of the type comprising an elongated, hollow conducting body inwhich a slot is formed, the longitudinal axis of the slot beingsubstantially parallel to the longitudinal axis of the hollow conductingbody. The hollow conducting body is usually of circular cross sectionbut it will be understood that the cross section of the conducting bodymay be of any one of a number of shapes such as square or rectangular.Aerials of the type specified are normally used to radiate horizontallypolarised waves, in which case the hollow body is arranged with itslongitudinal axis vertical. In one arrangement the electrical length ofthe slot is approximately half a wavelength long at the operatingfrequency and more than one such slot may be used, the several slotsbeing arranged in a vertical line. The object in using more than oneslot is to increase the power radiated in the horizontal directions andthus improve the aerial directivity. The same object can be achievedwithin limits by using a single longer slot but this is sometimesinconvenient from a mechanical point of view.

When the conducting body is a cylinder and it is required to produce asubstantially circular radiationfpattern in the horizontal plane it isusual to make the diameter of the cylinder equal to a small fraction ofa wavelength at the operating frequency. It is sometimes required,however, to provide a horizontal radiation pattern which is other thancircular. For example Where two transmitters are operating on the samefrequency or on closely spaced frequencies, it may become necessary, toavoid interference, to restrict the power radiated from at least one ofthe transmitters in the direction of the other without reducing thepower radiated in other directions. Furthermore, when the radiatedsignals occupy a large frequency band as in television, it is necessaryto maintain the aforesaid reduction in power in the same proportion overthe whole of the requisite frequency band.

In order to obtain a non-uniform horizontal radiation pattern it hasbeen proposed to make the diameter of the cylinder equal to a largefraction of the operating wave length. A disadvantage in using cylindersof large diameter is, however, that they are sometimes mechanicallyinconvenient and the number of variations obtainable in the radiationpattern is limited to a characteristic type. For example it is notpossible to obtain a radiation pattern of approximately cardioid shapewhich may be required when two transmitters interfere with. one anotheras previously described. The term cardioid is used to describe the shapeof a radiation pattern having a single sharply defined minimum value andnot a zero value as is usually understood in the case of, say, adirection finding aerial system.

It has also been proposed to use a cylinder whose diameter is smallcompared with the wavelength in use and to add flanges to the edges ofthe slot or slots. This arrangement suffers from the disadvantage thatthe range of variations obtainable in the radiation pattern is small forflange dimensions which are mechanically convenient and the radiationpattern varies in shape considerably with change in frequency especiallyin the directions of minimum radiation, which would result in distortionof radiated signals which occupy a wide band of frequencies.

One object of the present invention is to provide an improved slotaerial of the type specified which can be arranged to provide anon-circular radiation pattern whose shape remains substantiallyunchanged over a substantial band of frequencies.

Another object of the an improved slot aerial of the type specifiedwhose input admittance remains substantially constant over a substantialband of frequencies.

According to the present invention a slot aerial of the type specified,comprises two or more conductors disposed on or near the hollowconducting body, the longitudinal axis of the conductors being disposedsubstantially at right angles to the longitudinal axis of the conductingbody. The number, length and spacing of the conductors from one anotherand from the hollow conducting body is dependent upon the shape ofradiation pattern required. The conductors may be energised solely byincident radiation from the slot or they may be energised directly froma transmitter by way of a transmission line, the same transmitter beingused to energise the slot.

The radiation will now be described by way of example with reference tothe accompanying drawings, in which:

Figure 1 is a diagrammatic elevation of one embodiment of the invention,

Figure 2 is a cross-section taken at 22 in Figure 1,

Figure 3 is a diagrammatic elevation of a second embodiment of theinvention, and

Figure 4 is a cross-section taken at 4-4 in Figure 3.

In Figures 1 and 2 avertical metal cylinder 10 whose diameter isapproximately wavelength at the operating frequency has a straight slot11 formed therein parallel to the axis of the cylinder and about twowave lengths long. A transmitter shown diagrammatically at 12 isconcerned by a balanced feeder 13 to the centre of the slot, the twoconductors of the feeder being connected to the centres of the twolongitudinal edges respectively or" the slot.

Each of four horizontal rods 14 to 17 of insulating material has one ofits ends fixed to the cylinder 10 and the other ends of the rods 14 to17 carry four rods 18 to 21 respectively of conducting material. Theaxes of the four separating rods 14 to 17 lie in a common vertical planewhich includes the axis of the cylinder 10 and the centre'line of theslot 11 and the rods are equally spaced in that vertical plane by half awavelength at the operating frequency. The supporting rods are fixed tothe cylinder at four points respectively along the line on the cylinderwhere the said vertical plane cuts the cylinder. Each of the supportingrods 14 to 17 is a quarter of a wavelength long at the operatingfrequency.

The axes of the rods 18 to 21 fixed to the supporting rods 14 to 17 arealso horizontal and lie in a common vertical plane which is at rightangles to the plane containing the axes of the supporting rods 14 to 17.The junctions between the rods 18 to 21 and the supporting rods 14 to 17respectively are at the centres of the rods 18 to 21 respectively.

With this. arrangement the shape of the horizontal radiation pattern canbe varied substantially by varying the length of the rods 18 to 21. Whenthe length of the rods 18 to 21 is approximately equal to 0.6 of awavelength the radiation pattern is of the aforesaid cardioid shape andthe ratio of the maximum field strength of the present invention is toprovide minimum, field strength may be about 4:1. Furthermore theradiation pattern remains of substantially con: stant shape and theinput admittance of the aerial is of a form suitable for matching to atransmission line Within close. limits over a substantial band offrequencies for example. i4% of the mid-band frequency;

Referring now to the embodiment shown in Figures 3 and 4,. the cylinder10, transmitter 12 and feeder 13 are identical with those showninFigures l and 2.. The rods 14 to 21 of Figures 1 and 2 are, however,replaced by 8 rods of conducting material of which 4 are shown at 22 to25 in Figure 3 and a fifth is shown at 23 in Figure. 4. The rods inFigures 3 and 4. are arranged in coaxial pairs such as 23, and 23' inFigure 4 and are fixed to and project from opposite sides? of thecylinder. The. aXes f. the rodsv are horizontal and lie in a commonvertical plane which contains the. axis of. the cylinder and lies at.right angles to the plane containing the axis of the cylinder and thecentre line of the slot. The rods on each Side of, the cylinder are.spaced by half a wavelength and; each rod may have a length of 0.2 of awavelength. Such an arrangement is suitable for producing a horizontalradiation pattern of the same shape as that produced by aslottedcylinder whose diameter is a large fraction of the operatingwavelength. The. inconvenience of having a; large mechanical structureis however avoided.

Furthermore variations of input admittance with change in frequency overa substantial band of frequencies can be made small whereby asubstantially flat characteristic such as is required for televisiontransmission can readily be achieved. 7

It will be understood that other dispositions of conducting rods may beused to obtain radiation patterns of other shapes and that thecross-sectional shape of the slotted hollow body may be other thancircular. It is of course necessary to arrange the rods with their axessubstantially at right angles to the centre line of the slot.

The invention may be used in both transmitting and receiving aerials.

If in the arrangement of Figs. 1 and 2 it is desired to utilize theconducting rods as active rather than as passive elements, they may becoupled to the same feeder as the slot provided that a. suitableimpedance matching device is employed between the feeder and the rods.The construction of a suitable impedance matching device will beapparent to those skilled in the art.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

What we claim is 1. An aerial comprising a plurality of conductorsmounted ona hollow conducting body, including a slot extending along onesurface of the body, the conductors being of equal lengths and insulatedfrom the hollowbody, the mid points or the conductors lying along a linewhich is parallel to the longitudinal axis of the hollow body and is ina plane which includes the longitudinal axis of the hollow conductingbody and the centre line of the slot, and is spaced from the hollow bodyin a direction proceeding from the centre line of the slot through thelongitudinal axis of the hollow body, the

axes of the conductors being substantially at right angles 25 to thesaid plane.

2. A slotaerial according to claim 1, wherein the length of each of thesaid conductors is approximately 0.6 of a wavelength at the operatingfrequency, and the spacing from said hollow body is approximately 0.25of said wavelength.

References Cited. in the. file of this patent UNITED STATES PATENTS2,234,293 Usselman Mar. 11, 1941 2,510,290. Masters June 6, 19502,611,867 I Alford Sept. 23, 1952 OTHER REFERENCES Slot Antennas, by N.E. Lindenblad, Proceedings of the I. R. 13., vol. 35, No. 12, December1947, pages 1472-3 relied on.

